Presentation onLALR PARSER

( Look Ahead Parser )

Submitted To

Dharemendra Sir

Submitted By

Vivek Kr Poddar

Table Of Contento Introduction To LALR Parsero LALR Table Construction Methodo Examples – Related To Grammer, First, CLR, etc.o Computed LR( 0 ) Itemso GOTO Grapho Canonical Parsing Tableo LALR Parsing Table

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What is LALR Parsero LALR stands for (look ahead LR) parser.o LALR parser starts with the idea of building an LR parsing

tableo Tables generated by LALR parser are smaller in size as

compared to that of Canonical LR ( CLR ) and Simple LR ( SLR ) techniques.

o LALR parsers are slightly less powerful than LR parsers, but still more powerful than SLR parsers. 

o LALR is used by YACC and other parser generators because of its effectiveness.

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LALR Table Construction Ideao Construct the set of LR (1) items.o Merge the sets with common core together in CLR table.o If any problem arises then grammar is not LALR.

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ExampleGrammar:1. S’ -> S2. S -> CC3. C -> cC4. C -> d

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First : Where F Means First

F( S’ ) -> F( S ) -> F( C ) -> { c,d }

F ( C ) -> { c, d } -> c/d Look Ahead

Symbol

Note :: CC are two different items.

Note :: Non-terminals denoted by upper-case letters, terminals denoted by lower-case letters

S’ -> .S,$ :: This (a rule with a dot in it) is called an item, it indicates what is in the stack [ left side of . ] and what is to be expected on input [ right side of . ]Anything after ( , ) comma is termed as look ahead.

Computed LR ( 0 ) & LR ( 1 ) Items 6

I5: S -> CC., $

I6: C -> c.C, $ C -> .cC, $ C -> .d, $

I7: C -> d., $

I8: C -> cC., c /d

I9: C -> cC., $

I0 : S’ -> .S, $ S -> .CC, $ C -> .c C, c /d C -> .d, c /dI1: S’ -> S., $I2: S -> C.C, $ C -> .c C, $ C -> .d, $ I3: C -> c. C, c /d C -> .Cc, c /d C -> .d, c /d14: C -> d., c / d

Grammar:1. S’ -> S2. S -> CC3. C -> cC4. C -> d

Grammar:1. S’ -> S2. S -> CC3. C ->

cC/d First ( c/d )

Note : I0 consist of LR ( 0 ). Items while rest are LR ( 1 ) items.

Its always suggested not to work beyond State 2 by hand as they are compicated and should be calculated using standard tools.

7 Goto Graph

8 Canonical Parsing TableStates

c d $ S C 0 S3 S4 1 21 acc2 S6 S7 53 S3 S4 84 R4 R45 R26 S6 S7 97 R48 R3 R39 R3

Actions GOTO

9 LALR ParserActions GOTO

Merge the Cores:

What is core ?A core is a set of LR (0) (SLR) items for the grammar.

o 3 & 6o 4 & 7o 8 & 9

10 LALR Parsing Table

ActionsStart

c d $ S C 0 S36 S47 1 21 acc2 S36 S47 5

36 S36 S47 8947 R47 R47 R475 R2

89 R36 R36 R36

GOTOAction

Conflicts In LALR Parsero LALR Parser cannot introduce shift/reduce conflicts.o Such conflicts arises when the look ahead is same as the

token on which we can shift.o They depend on the core of the item but we merge only

those rows which have common cores.o The only way by which this conflict can arise in LALR is when

the conflict is already their in the LR(1).

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11 Refrence

ActionsGOTOActionUsed to generate First of Given Grammer

http://hackingoff.com/compilers/predict-first-follow-set

IITKGP notes for making presentationhttp://www.facweb.iitkgp.ernet.in/~niloy/COURSE/

Compilers Principles Techniques and Tools (2nd Edition) - BOOK

Thank YouFor Listening

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